In recent years, the demands for small-size, high-capacity information recording/reproducing device (memory device) have been rapidly expanding. Among them, NAND flash memory and a small-size HDD (hard disk drive) have rapidly evolved in recording density and created a large market. Under these circumstances, several ideas for novel memory have been proposed in an effort to significantly exceed the limit of memory density. Among them, non-volatile semiconductor memory, which employs a variable resistance material having a low resistance state and a high resistance state, is attracting attention.
Non-volatile semiconductor memory includes a memory cell array in which a plurality of memory cells are arranged at intersection points between a plurality of bit lines and a plurality of word lines, and the recording density of the memory cell array is expected to increase with the miniaturization of the memory cell.
Meanwhile, when memory cells are debugged, it is often difficult to specify a defective part on a circuit by only output from a pad of a chip. In order to make it easy to debug, a waveform of a signal line such as a bit line or a word line needs to be observed if at all possible. To this end, metallic pieces electrically connected to signal lines are formed on a top interconnection layer. A hole is formed to make a metallic piece to be connected to a signal line of a defective bit to be selectively exposed by a focused ion beam (FIB) when a failure occurs. A probe electrode is formed by filling the hole with a conductive material. A needle of a pico-probe comes into contact with the probe electrode, and then a waveform of the signal line is observed.
However, a space of a bit line or a word line is reduced with the miniaturization of the memory cell, and thus it becomes difficult to form a metallic piece for observing a waveform of a signal line such as a bit line or a word line.